(1) Field of The Invention
The present invention relates to writing servo signals on a servo disk in a magnetic disk apparatus.
The data heads which write and read data are regularly positioned on a designated track on a data surface of a data disk by reading the servo signals previously written an the servo disk. This is called closed loop servo control. Servo signals are usually written on a surface, which is called a servo-surface, of a magnetic disk. As one example of closed loop servo control, there is a servo control system having a servo disk which is exclusively used for servo control. This control system is called simply called "servo-surface servo", hereinafter. There is another control system which is also an example of closed loop servo control and is performed using servo data which are stored in a data surface of a data disk instead of using a servo disk as in the case of "servo-surface servo". This control system is called simply "data-surface servo", hereinafter. However, the "servo-surface servo" has been the type of control system used in most magnetic disk apparatuses for a long time.
The present invention can be applied to the "servo-surface servo" type control system.
(2) Description of The Related Art
The data heads in a magnetic disk apparatus are position-controlled so as to be on-track on a data disk during writing or reading, based on the way in which a servo head reads servo signals stored on a servo disk in "servo-surface servo". FIGS. 1 (a) to 1(e) show a group of charts illustrating waveforms successively produced when a servo head reads the servo data stored on the servo disk. Therefore, the transversal direction of this chart represents the direction of circumference of a track and the longitudinal direction of this chart represents the radial direction of the servo disk. In FIGS. 1(a) to 1(e), each chart of a waveform is schematically expressed using triangle wave for simplicity. The group of charts of waveforms (a),(b),(c) and (d) correspond to what are called the normal bits and are produced from adjacent four tracks, respectively. Namely, waveforms (a), (b), (c) and (d) correspond to track 1, track 2, track 3 and track 4 in the data disk, for instance. The waveform (e) corresponds to what is called the index bit which is written in a portion of each track. The signals indicated by CB are clock bits to be used as the standard clock in demodulation of the servo signals and in data writing. The signals indicated by P1 and P2 are positional bits. The positional bit P1 is expressed by two kinds of bit named ODD1 or EVEN1 and the positional bit P2 is likewise expressed by ODD2 or EVEN2, according to their position. Further, the waveform (e) is expressed by a combination of ODD1 and EVEN2 in this case. These four kinds of bits are written periodically in the tracks of the servo disk.
The positioning of the data head on a track of the disk is performed by reading the servo signals from the servo disk by the servo head, demodulating the positional bits (P1 and P2) and detecting the peak level of the P1 and P2.
In the "servo-surface servo", the servo signals are read from the servo disk and the positional bits are demodulated so that the data head is positioned, even when the data head is writing data on the data disk. At that time, electromagnetic noise is generated near the data head. Particularly, noise of a high level is generated when the data is being written as compared with when the data is being read. The high level noise disturbs the demodulation of the positional bits. As the result, the correct reading the servo data becomes impossible. The disturbance by the electromagnetic noise generated from the data head which is nearest to the servo head is most remarkable. The influence of the electromagnetic noise on the demodulating the positional bits is further enhanced due to the following reason. The standard clock for writing data on the data disk (write clock) is usually produced by a phase lock oscillator which is synchronized to the clock for reading the servo signals (servo clock) in order to eliminate the influence of fluctuation of rotational speed of the magnetic disk on detecting the positional bit signals. Therefore, the relationship of phase between the write clock and the servo clock is invariable. As a consequence, the servo signals are synchronized to the electromagnetic noise.
A conventional method for eliminating the influence of electromagnetic noise generated from the data head is to shield the servo head electromagnetically by a shield. However, a space large enough to provide the effective shield is lost as the magnetic disk apparatus becomes smaller according to a recent trend of miniaturization. Moreover, the synchronization between write clock and servo clock becomes indispensable since the magnetic disk apparatus having a high recording density has become common.
Another method for eliminating the influence of electromagnetic noise is published in Japanese laid-open patent publication SHO 58-97164 entitled "A magnetic disk apparatus" by S. Sengoku, Jun., 9, 1983. The method disclosed utilizes a signal processing technique instead of utilizing an electromagnetic shield. However, what is disclosed relates to adjustment of a phase difference between the data writing clock and the servo signal. This is not applicable for the method of writing the servo signals on the servo disk.
A method of writing the servo signals on the servo disk is disclosed in Japanese laid-open patent publication SHO 61-26922 entitled "A magnetic disk apparatus" by T. IWAI and K. NISHIMURA, Feb. 6, 1986. In this method, the electromagnetic noise appears symmetrically on the two positional bit signals which are used for demodulation of the servo signals. For this purpose, the time difference between the two positional bit signals must be an even multiple or about even multiple of period of the write bits. This condition is hardly satisfied for arbitrary stored servo data pattern. That is, the method disclosed is effective for only special servo data patterns to be written. Therefore, a method of eliminating the influence of the electromagnetic noise generated from the data head during writing, while the servo head is reading the arbitrary stored servo data pattern is required.